1) Field of the Invention
This invention relates to a method for adding supercritical carbon dioxide to a molten thermoplastic resin and also to a process for producing an expanded thermoplastic resin product by making use of the addition method. More specifically, the present invention is concerned with a process for producing expanded thermoplastic resin products of uniform quality by using carbon dioxide as a blowing agent.
2) Description of the Prior Art
For the production of expanded thermoplastic resin products, processes making use of a chemical blowing agent or gas blowing agent are known. A chemical expanding process generally comprises mixing raw material pellets with an organic blowing agent of a low molecular weight, which decomposes at a molding temperature to produce gas, and then heating the resulting mixture in an extruder to a decomposing temperature of the blowing agent or higher to effect expansion molding. According to this process, the decomposition temperature can be easily adjusted by adding an expanding aid or the like. Moreover, this process can obtain expanded products having relatively uniform closed cells. However, in addition to high cost, these expanded products tend to develop discoloration, offensive odor, food sanitation problems and the like due to decomposition residues of the blowing agent, said decomposition residues still remaining in the expanded products, and an undecomposed portion of the blowing agent. There are other problems including smearing of molding machines caused by a chemical blowing agent and defective molding associated with such smearing.
On the other hand, a gas expanding process making use of a physical blowing agent is a process which comprises melting a resin in a molding machine, feeding an organic compound of a low boiling point such as butane, pentane or dichlorodifluoromethane to the resin, kneading the resin and the organic compound together, and then releasing the resulting mixture into a low-pressure zone to effect expansion molding. The organic compound of the low melting point, which is employed in this process, has compatibility with the resin and is hence excellent in solubility and also in retention, so that it features the availability of expanded products of high expansion ratios. Nonetheless, such blowing agents are costly and moreover, have dangers such as inflammability and toxicity. They also have a potential problem of air pollution. Further, there is a move toward the total ban of flon-series gases led by dichlorodifluoromethane in view of environmental problems such as destruction of the ozonosphere.
With a view to overcoming such problems of the conventional processes, numerous processes making use of an inert gas such as carbon dioxide gas or nitrogen as a blowing agent, said inert gas being clean and economical, have been proposed. However, the inert gas has poor solubility in a resin because of its low compatibility with the resin. Expanded products have large and uneven cell diameters and low cell populations, leading to problems in external appearance, mechanical strength and heat insulating properties. Further, no method has been established yet for the stable addition of an inert gas into a molding machine. Uneven expansion therefore occurs on products, thereby making it difficult to obtain expanded products of uniform quality.
When an expanded thermoplastic resin product is produced using an inert gas, especially carbon dioxide, it has been the conventional practice to add the gas directly from a gas cylinder via a pressure reducing valve. According to this method, however, fluctuations take place in the flow rate of the blowing agent due to fluctuations in the pressure of a resin in a blowing agent adding section. As a result, uneven expansion occurs on products, thereby making it impossible to obtain expanded products of uniform quality. In addition, this method cannot add the blowing agent if the pressure of the resin in the blowing agent adding section is higher than the pressure of the gas cylinder.
U.S. Pat. No. 5,158,986 discloses a technique for obtaining an expanded product by using a supercritical fluid as a blowing agent and impregnating a thermoplastic resin with the supercritical fluid. As a supercritical fluid has excellent solubility similar to that of a liquid and superb diffusibility close to that of a gas, it shows high solubility and diffusion rate in a resin. The resin can therefore be impregnated with the blowing fluid in a short time. Two processes are proposed for obtaining expanded products in this patent publication, one comprising forming a thermoplastic resin into a sheet through an extruder, introducing the sheet into a pressurized chamber filled with carbon dioxide in a supercritical state to impregnate the sheet with carbon dioxide, and then heating the sheet by a heater in an expanding chamber under atmospheric pressure to cause the sheet to expand; and the other comprising melting a resin in an extruder, impregnating the molten resin with carbon dioxide in a supercritical state, extruding the resulting impregnated resin into a sheet-like product, introducing the sheet-like product into a pressurized chamber to form cell nuclei owing to the pressure difference, and then heating or cooling the resultant sheet.
These processes however require large-scale high-pressure facilities and hence an enormous initial cost and are poor in work efficiency, so that they can be hardly practiced on an industrial scale. Further, the former process requires a long time for the full impregnation of the sheet-like product with carbon dioxide because the sheet-like product is directly impregnated. On the other hand, the latter process impregnates carbon dioxide into the molten resin so that the penetration speed of carbon dioxide in the latter process is faster than that in the former process. It is however difficult to achieve solubilization of carbon dioxide through kneading in only one extruder.
The present inventors proposed in Japanese Patent Application No. 185268/1997 (Japanese Patent Laid-Open No. 76560/1998) a process for the production of an expanded thermoplastic resin product by expansion extrusion, in which carbon dioxide and/or nitrogen in a supercritical state is used as a blowing agent.
In the above invention, two methods are exemplified for mixing the blowing agent into a melt of a resin composition in a continuous plasticator, one being to inject gaseous carbon dioxide and/or nitrogen under compression with a gas, and the other to inject liquefied carbon dioxide and/or nitrogen by a plunger pump. These methods can perform, in a simple step and with simple facilities, the addition of carbon dioxide in a supercritical state into an extruder, said addition being practically unfeasible in any manner applicable for industrial production when the technique of U.S. Pat. No. 5,158,986 is relied upon.
Through a further investigation by the present inventors, it has become increasingly clear that the amount and pressure of carbon dioxide, which is delivered from a compressor pump, fluctuate depending on the temperature around the compressor pump and the temperature of carbon dioxide to be injected into the compressor pump. Incidentally, the above invention makes no mention about the production of an expanded product the expansion ratio of which exceeds 10 times.
As a further method for adding a blowing agent under a pressure equal to or higher than its critical pressure, a process for obtaining an expanded thermoplastic resin product is proposed in Japanese Patent Laid-Open No. 222922/1989. According to this process, the pressure of an inert gas is adjusted via a reducing value to fall within a range not lower than the pressure of a molten resin in a gas addition section but not higher than 9.8 MPa, and is then injected into an extruder. However, this process cannot add the blowing agent either if the resin pressure is 9.8 MPa or higher. It is therefore necessary to control the pressure of the molten resin at 9.8 MPa or lower in the gas adding section. Significant limitations are therefore imposed on the usable resin, the extruder and extrusion conditions, so that expanded products available by this process are substantially limited. When carbon dioxide is used as a blowing agent, its addition at 9.8 MPa or lower into the extruder is accompanied by a limitation on a maximum amount to which the blowing agent can be added. Products of a high expansion ratio are not available accordingly. Further, the solubility of carbon dioxide in the molten resin is poor, and a substantial time is required until dissolution. Expanded products available from this process have large cell diameters, uneven cell distribution, and small cell populations.
For the provision of an expanded thermoplastic resin product, a still further process is proposed in Japanese Patent Publication No. 41161/1994. According to this process, pressurized carbon dioxide is once stored in a tank with its temperature maintained at its critical temperature or higher, and the pressurized carbon dioxide is then reduced in pressure and is charged at a pressure of 9.8 MPa or higher into an extruder while controlling its flow rate.
This process is however accompanied by a limitation on a maximum amount to which carbon dioxide can be added. The patent publication contains a statement to the effect that carbon dioxide cannot be added stably into the system if its proportion exceeds 2 wt.%. Therefore, any attempt to obtain a product of a high expansion ratio leads to occurrence of uneven expansion on the product, thereby making it difficult to obtain expanded products of uniform quality. Moreover, this process requires large-scale and complex facilities, so that a significant initial cost and a wide installation site are needed. This process also involves a still further problem in that the flow rate of carbon dioxide is hardly controllable.
As has been described above, use of carbon dioxide as a blowing agent has heretofore been difficult in stably adding a predetermined amount of carbon dioxide to a molten thermoplastic resin in a forming machine. It has therefore been difficult to obtain expanded products of uniform quality, especially to produce expanded products of a high expansion ratio with uniform quality.